I got in solo as well within an hour of the worlds going up, even though lots of people were there on my world alone.
I think it is just like the chocobo races where it tries to bring 24 people into an instance, but if it doesn't manage it after a minute or so then it just starts anyway. So if there are 25 people then you could get into one on your own. I'm not certain why others wouldn't join in that time when it's new content, so maybe each instance also sort of rotates how relevant it is like Eureka and Bozja do.
One time in Eureka for example the instance locked and I was in there alone for 3 hours, and near the end of it, it opened up again and everyone joined saying "fresh instance".
